The invention relates to an image correction method for a flat dynamic X-ray detector which includes a sensor matrix whose photodiodes are preceded by a scintillator for converting the X-rays into light and succeeded by a light source for uniform illumination of all photodiodes. The invention also relates to an X-ray device.
Flat dynamic X-ray detectors (FDXD) are used notably in medical diagnostic X-ray devices. The construction of flat dynamic X-ray detectors is described, for example in EP 034 54 A2 as well as in EP 0 440 282 A2.
For the image correction method in accordance with the invention it is necessary that the X-ray detector includes a light source for uniform backlighting of all photodiodes of the X-ray detector. The installation and operation of the backlighting arrangement are known per se and are described, for example, in WO 98/01992.
For the flat dynamic X-ray detectors that are based on amorphous silicon it is known that image information from previous X-ray exposures may still be visible in later X-ray images. A remainder of the preceding image or the preceding images can then still be observed in the instantaneous X-ray image which will be referred to hereinafter as the bright image. Such undesirable image artefacts decay in time and are referred to hereinafter in general as after-image effects. Thus far the after-image effects are attributed to residual signals from the scintillator (afterglow) and from the photodiode. However, the incomplete reading out of the charge from the photodiode in the read-out amplifier is also known as a cause of residual signals. In a physical sense the so-called trapping and subsequent de-trapping of charge carriers in the scintillator and in the photocathode are the sources of the residual signal effects. For example, the charge carriers produced during an illumination of the photodiode are bound partly to trapping points (trapping). At a later instant the trapped charge carriers are released (de-trapping) again, thus leading to a delayed image signal and hence to a residual signal effect.
In order to eliminate such undesirable residual signal effects, DE 196 31 624 discloses an X-ray diagnostic device which includes a correction unit that detects any residual signals from at least one dark image. The residual signal component in the subsequent bright images can be determined and eliminated from said dark images in which only the instantaneous residual signal can be observed.
Experiments, however, have shown that after-image effects still occur in the bright images despite the dark image correction.
On the basis of this state of the art, therefore, it is an object of the invention to provide an image correction method for a flat dynamic X-ray detector of the kind set forth which enables improved correction of the after-image effects. It is also an object of the invention to propose an X-ray device which carries out said improved image correction method.
The solution to the described problem is based on the idea that in addition to the known residual signal effects there are further after-image effects which are not visible in pure dark images and hence cannot be eliminated by means of the known correction method. Such further after-image effects are so-called xe2x80x9cgain effectsxe2x80x9d which occur in the scintillator as well as in the photodiode. Tests have demonstrated the existence of such gain effects for scintillators and photodiodes. It has been found that for customary diagnostic X-ray doses the gain effect in the scintillator is insignificant and hence need not be taken into account for the image correction method. The gain effect in the photodiode, however, may give rise to an increase of the gain by a few percents, and hence to visible after-image effects in subsequent bright images, already in the case of customary diagnostic X-ray doses.
Such gain effects can again be attributed to the trapping of charge carriers, because trapping points that are already occupied cannot accommodate further charge carriers. As a result, during the instantaneous bright image exposure the image signal increases in regions in which large numbers of trapping points are still occupied because of a preceding (X-ray) exposure, because fewer of the charge carriers produced can be bound to trapping points. The resultant increase of the image signal becomes manifest as a gain effect, but not directly as a residual signal effect.
The invention utilizes the fact that the gain effect in the photodiode can be made visible in an exposed image so that the strength of the gain effects in the photodiodes can be determined pixel by pixel. More specifically, the gain effect for each matrix cell of the sensor matrix is determined in conformity with claim 1.
The pixel values of the instantaneous bright image that have been corrected in accordance with the invention can then be further processed in known manner, for example by filtering, storage or introduction into networks.
Calculating the strength of the gain effects from at least one light source image and one light source reference image in accordance with the invention enables the formation of images with a significantly lower dose even after exposures with a high X-ray dose, such images formed with a lower dose nevertheless being free from the described gain effects to a high degree.
Because X-ray devices provided with a flat dynamic X-ray detector often also include a light source on the rear, the X-ray device does not require structural modifications so as to implement the image correction method in accordance with the invention.
In order to eliminate also the other after-image effects, simultaneously with the image correction method in accordance with the invention there is performed at least one further, known image correction method such as, for example, the previously described dark image method.